1. Technical Field
The present invention relates to an ultrasound diagnostic apparatus, and more particularly to image forming technology in which bubbles are used.
2. Related Art
Microbubbles (nanobubbles) as used herein refer to fine air bubbles injected into a liquid and so on. Due to various excellent characteristics, microbubbles are being utilized in many technical fields. In application to the medical field, for example, microbubbles, which can serve as a preferable reflector of ultrasound, are being utilized as a contrast agent for forming an ultrasound image.
In ultrasound image formation using a contrast agent, technologies in which a harmonic component obtained from bubbles is focused are known, of which phase inversion technology (a pulse inversion technology) and power modulation technology are especially well known. Obviously, there exist a large number of technical documents concerning the image forming technology focusing on the harmonic components, including JP 2005-152177 A (Patent Document 1), JP 2001-258882 A (Patent Document 2), and JP 2006-271559 A (Patent Document 3), for example. Here, it should be noted that neither Patent Document 1 nor 2 describes the technology in which three-dimensional harmonics obtained from bubbles are positively imaged. Further, Patent Document 3 describes landmark technology in which the principle of the phase inversion technology is applied to form an image of a harmonic component by using two transmission signals having phases that are inverted with respect to each other.
In the case of forming an image of microbubbles (a contrast agent) which are injected into a relatively thick blood vessel and so on, it is possible to achieve a relatively high contrast by using conventionally known technology, because blood, in which scattering of ultrasound or the like is unlikely to be observed, is present around the microbubbles. On the contrary, when observing microbubbles surrounded by a capillary and a lymph node such as tissue and a tumor, because real tissue is present around the microbubbles, it is more difficult to achieve a high contrast than when blood exists around the microbubbles.
Further, a contrast agent suitable for imaging using ultrasound at a low-middle sound pressure (e.g. “Sonazoid” (registered mark)) has recently drawn attention, and highly sensitive imaging technology using ultrasound at a low-middle sound pressure is also desired.
Under such circumstances, the present inventors have continued to research and improve imaging technology by ultrasound in which bubbles (e.g., microbubbles) are utilized, particularly the behavior of bubbles to which ultrasound has been applied. The present inventors have also continued to research and improve technology of extracting a harmonic component obtained from a target subject such as bubbles.